1. Field of the Invention
The invention pertains generally to optical systems, e.g., optical communication systems and optical computers.
2. Art Background
Optical systems, e.g., optical communication systems and optical computers, are in use, or are being developed, because such systems are capable, or offer the possibility, of transmitting and/or processing much larger amounts of information, much more quickly, than is possible using purely electronic systems.
The optical systems, referred to above, typically include one or more sources of electromagnet radiation, e.g., one or more semiconductor lasers, a device (or devices) for processing the electromagnetic radiation emitted by the sources, and one or more detectors for detecting the processed electromagnetic radiation. The electromagnetic radiation emitted by the sources is communicated to the processing device (or devices), and then to the detectors through, for example, the air, a vacuum, or through waveguides such as optical fibers.
The processing devices employed in the optical systems include, for example, switches (devices for switching electromagnetic radiation from one waveguide to another waveguide of the system), amplitude modulators (devices for altering the intensity of the electromagnetic radiation), and multiplexers/demultiplexers (devices which serve, for example, to redirect the electromagnetic radiation carried by a plurality of waveguides onto a single waveguide and vice versa). At present, these devices are typically fabricated in electro-optic materials, such as lithium niobate, and the processing is performed electronically. For example, a switch, containing two or more planar waveguides (typically in optical communication with other waveguides, e.g., optical fibers, of the system) is formed in a lithium niobate substrate by depositing a dopant, such as titanium, onto the surface of the substrate in the pattern desired for the planar waveguides. The substrate is then heated to diffuse the dopant into the substrate, thus forming the planar waveguides. In addition, electrodes are formed on opposite sides of one or more of the planar waveguides. To produce switching, a voltage is applied across the electrodes, thus inducing a local change in the optical polarizability of the lithium niobate, which locally changes the refractive index and, in turn, alters the path of the light from one planar waveguide to the other planar waveguide. Significantly, switching speed depends upon the configuration of the electrodes. For example, when applying a voltage to electrodes having a lumped electrode configuration, the time required to achieve switching is limited by the RC time constant of the electrodes, which is typically several nanoseconds (ns). (Removing the applied voltage results in the induced changes in optical polarizability and refractive index disappearing essentially instantaneously.) As a consequence, the cycle time (the time interval between successive switching operations) is limited to (is no smaller than) several nanoseconds (ns), and thus the reptition rate (the number of switching operations per unit time, equal to the inverse of the cycle time) is limited to no more than about 3.times.10.sup.8 Hertz (Hz). On the other hand, when using a traveling wave (transmission line) electrode configuration, a switching time, and a cycle time, equal to several hundreds of picoseconds (ps) is readily achieved, yielding a repetition rate as high as 3.times.10.sup.9 Hz.
To achieve even shorter cycle times and higher repetition rates, devices have been sought, and proposed, in which the processing is achieved by purely optical means. That is, these proposed devices typically include a region of material which serves as a transmission medium for a beam of electromagnetic radiation (hereafter the signal beam) to be processed. Significantly, the transmission medium is chosen to exhibit a nonlinear optical response (a change in refractive index and/or optical absorption) when impinged by a second beam of electromagnetic radiation (hereafter the control beam), with the nonlinear optical response serving to effect the processing of the signal beam.
One device which is exemplary of the devices employing purely optical processing is disclosed in J. L. Jewell et al., "Parallel Operation and Crosstalk Measurements in GaAs Etalon Optical Logic Devices," Applied Physics Letters, Vol. 48, No. 20, May 19, 1986, pp. 1342-1344. This device is a gallium arsenide (GaAs) etalon which includes a layer of GaAs (an inorganic semiconductor material), having a thickness less than about 1 micrometer (.mu.m), sandwiched between two dielectric mirrors. The mirrors are designed to exhibit relatively high reflectivity to electromagnetic radiation having a wavelength of about 890 nanometers (nm), and relatively low reflectivity to electromagnetic radiation having a wavelength of about 800 nm. Moreover, the etalon is designed so that a peak in the transmission curve of the etalon occurs at the former wavelength. In operation, and in the absence of a control beam, a signal beam, having a wavelength of 890 nm, impinging upon the GaAs, suffers relatively little absorption, and is thus largely transmitted. On the other hand, by impinging a control beam, having a wavelength of 800 nm, upon the GaAs, valence band electrons in the GaAs are promoted into the conduction band, which alters the refractive index of the GaAs and, as a consequence, (essentially instantaneously) shifts the etalon transmission peak away from 890 nm. As a result, the signal beam (having a wavelength of 890 nm) suffers relatively low transmission. Depending upon the thickness of the GaAs, this decrease in etalon transmission exhibits a characteristic decay time, .tau. (the time interval over which the decrease in transmission decays to 1/e of its original value, after the control beam is turned off), ranging from about 200 ps to about 15 ps. Significantly, it is this decay time which limits device speed. That is, the control beam cannot be turned on a second time to produce a significantly decreased etalon transmission until the first transmission decrease has largely disappeared, which typically takes about 2.tau.. Thus, and depending upon the thickness of the GaAs, the cycle time of this device (the time interval between successive, low transmission states) ranges from about 400 ps to about 30 ps, and therefore the repetition rate ranges from about 2.5.times.10.sup.9 Hz to about 3.3.times.10.sup.10 Hz.
Rather than employing inorganic semiconductor materials, such as GaAs, processing devices have also been proposed which employ organic materials, such as polydiacetylene (PDA). In this regard, it is known that one particular crystalline form of PDA, known as poly-2,4-hexadiyn-1,6-diol bis (p-toluene sulfonate) (PDA-PTS), exhibits the absorption spectrum depicted in FIG. 1, which includes a wavelength region of relatively high absorption (a region where the absorption coefficient, .alpha., is greater than or equal to about 2.times.10.sup.4 cm.sup.-1), extending from about 640 nm to shorter wavelengths. It is also known that this relatively high absorption region is associated with electrons being promoted from a relatively low energy electronic state (the ground state) to a higher energy electronic state as a result of photon absorption. Significantly, it has long been known that PDA exhibits a relatively large (compared to that in other materials) nonlinear optical response at wavelengths associated with relatively low absorption, i.e., wavelengths at which .alpha. is less than about 2.times.10.sup.4 cm.sup.-1. This response is believed to decay essentially instantaneously, i.e., within 10.sup.-15 seconds. Unfortunately, the magnitude of this response (though relatively large compared to that in other materials) is generally too small to be useful for device applications. On the other hand, it has long been believed that the (much larger) nonlinear optical response at wavelengths exhibiting relatively high absorption (.alpha. is greater than or equal to about 2.times.10.sup.4 cm.sup.-1) would have a much longer decay time.
An experimental investigation into the decay times associated with the nonlinear optical responses produced in PDA-PTS at wavelengths ranging from about 651.5 nm (which is at the long-wavelength edge of the relatively high absorption region extending from about 640 nm to shorter wavelengths) to about 701.5 nm has been carried out and is described in G. M. Carter et al, "Time and Wavelength Resolved Nonlinear Optical Spectroscopy of a Polydiacetylene in the Solid State Using Picosecond Dye Laser Pulses", Applied Physics Letters, Vol. 47, No. 5 (Sept. 1, 1985), pp. 457-459. This investigation involved the use of conventional, degenerate four-wave mixing. That is, two pulses of electromagnetic radiation, of identical wavelength, were interfered within a volume region of the PDA-PTS to produce a periodic intensity variation which resulted in a periodic variation in refractive index (the nonlinear optical response) and thus, in effect, a volume diffraction grating. These pulses had durations of about 6 ps and had peak intensities of about 2.5.times.10.sup.7 watts per square centimeter (W/cm.sup.2). A third pulse of electromagnetic radiation, of identical wavelength, duration and intensity was impinged upon the PDA-PTS and diffracted by the diffraction grating to produce a fourth beam of electromagnetic radiation which traversed the thickness of the PDA-PTS and was then detected by a detector. By varying the arrival time of the third pulse relative to the first two pulses, it was determined that the duration of the nonlinear optical response at the edge of the PDA-PTS relatively high absorption region was less than the pulse duration, and thus less than about 6 ps. Based upon this experimental data, and by making a number of (implicit) assumptions, it was then inferred that the nonlinear optical response at relatively high absorption wavelengths, e.g., at 625 nm, would also have a decay time less than about 6 ps. One of the (implicit) assumptions underlying this conclusion is that the absorption in PDA-PTS is attributable to the existence of only a single electronic excited state, and it was this state that was accessed (populated) by the two interfering pulses.
To date, there has been no reported experimental verification that PDA truly exhibits a nonlinear optical response at relatively high absorption wavelengths having a decay time less than about 6 ps. In the absence of such verification, the utility of PDA in fast optical processing devices is still in question.
Thus, those engaged in the development of optical processing devices have sought, and continue to seek, fast devices capable of achieving relatively high repetition rates.